The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its advantages to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an express or implied admission that such art is widely known or forms part of common general knowledge in the field.
The discharge of slurry as an underflow stream from below a separator is not easy to control. The slurry normally contains particles less than 1 mm, but could equally contain larger particles, or a very small portion of these larger particles. The separator typically has a valve that can be partially opened to regulate the slurry discharge. Thus, as the valve is gradually opened there is a very significant increase in the discharge rate of the slurry. This rapid discharge arises because the slurry in the separator, located above the valve, delivers a significant hydrostatic head, whereas the opening created by the valve is exposed to atmospheric pressure. The pressure driving force of the discharge slurry is therefore significant.
It is standard practice to apply a PID control strategy to regulate this discharge according to some objective. In separators like a reflux classifier or a teetered bed separator the lower zone of the vessel is fluidised via an upward current fluidising flow. This results in a suspension density bed profile, which can be measured using pressure transducers. Usually two pressure transducers are located at two elevations or heights of the separator, thus providing the average suspension density in the zone between those elevations. The separator is then operated by controlling the underflow discharge in order to target a specific suspension density set point. This approach tends to deliver a corresponding underflow yield and underflow grade.
There are many kinds of valves that are used with these types of separators. However, it is common for the valve opening to vary non-linearly, while the discharge rate varies considerably. In addition, the coarser particles can easily bridge the gap of the opening, limiting discharge out of the valve opening, thus causing the controller to seek an even larger opening by further opening the valve. Once this bridging breaks, the rate of discharge increases very rapidly. For these reasons, it is easy for the valve to be open too wide and for too long, causing excessive and rapid discharge. As a consequence, the suspension density rapidly falls below the set point. The valve is then forced to close to allow the suspension density to rise up back towards the set point. Thus, the system can often cycle between these two extremes of rapid slurry discharge leading to a rapid fall in the suspension density below the set point and reduced slurry discharge to bring the suspension density back to the set point. This cycling also makes it difficult to accurately target low set point densities near the suspension density of the feed.
The problem described above is acute for relatively small vessel cross-sections. In this case, the size of the valve must be large compared to the size of the vessel in order to allow the free passage of the coarser particles through the valve. Failure to do so will result in blockage and hence failure of the valve. When the valve is fully open, this can lead to the entire separator emptying very quickly.
Full scale industrial separators tend to have a valve size much smaller than the tank or vessel cross-section. Although relatively small, the valves are much larger than the coarsest particles, so bridging is not necessarily a problem. Nevertheless, these large vessels can also suffer from the problems described above, forcing the suspension in the zone above the valve to discharge too quickly with undesirable results. Rather, an orderly movement of material towards the underflow is essential in order to maximize the separation efficiency.